Power supplies are ubiquitous devices present in electrical and electronic equipment. Typically, a power supply converts alternating current (AC) power into direct current (DC) power for use within the equipment. The AC power is generally delivered to the power supply at a relatively high voltage, for example, 120 VAC, while the DC power is generated within the power supply at one or more relatively low voltages, for example, 5 and 12 VDC. In some applications, the power converted by a power supply is received from a DC source, but at a voltage that cannot be used directly by the equipment. For example, the input power may come from a source of voltage that is too high or too low for direct use within the equipment. The power supply then regulates the voltage down to the needed level, or performs DC-to-DC transformation, either stepping the DC voltage up or down, as needed.
Ideally, the DC voltage delivered by a power supply is stable and does not have any AC components. In practice, however, the DC voltage has some AC components. The most common source of the AC components is feed through of the AC voltage, such as the 60 Hz spectral components in North America or the 50 Hz frequency common in Europe. Another source of AC noise is the equipment using the DC power. Still another source of the noise is radio frequency interference. But whatever the source of the AC noise on the power supply output, it is desirable to reduce its magnitude. A power supply's ability to suppress the AC noise on its output is an important performance characteristic of the supply.
Another important measure of power supply performance is the capability to continue delivering stable DC power during disturbances on the AC power line that feeds the power supply. This capability is sometimes descriptively called “ride-through” capability, because it allows the equipment to perform as expected during AC power interruptions of short duration, or to power down in a controlled manner during such interruptions.
Large capacitors are often connected across DC power supply outputs to improve both AC noise suppression and ride-through capability. Capacitors perform these functions because they are reservoirs of electrical charges, and can absorb or supply the charges as required. The larger the capacitance of a given capacitor, the better it will suppress AC noise and the longer it will be able to supplement or replace DC power normally provided by the power supply. One type of capacitor that can provide large capacitance is that known to those skilled in the art as a double-layer capacitor. Double layer capacitors can provide previously unattainable large capacitance values in small form factor housings. For example, a 500 Farad double-layer capacitor can now be made to fit within a battery sized housing, including D-cell sized housings and the like.
Connecting a capacitor across a power supply output is not without its own set of problems. In the present context, we focus on three such problems. First, a capacitor may draw a large amount of electrical current on power-up, until the capacitor is sufficiently charged. This is problematic because the capacitor may keep the voltage of the power supply from reaching its nominal level for an excessive period of time. Power monitoring and power-on reset circuits, common in electronic equipment, may time-out before the voltage stabilizes at the nominal level, keeping the equipment in the reset mode or initiating another start-up sequence of the equipment. Even when the equipment can tolerate a prolonged start-up period, many users find additional waiting annoying. These problems become worse as capacitance is increased, because higher capacitance allows a capacitor to receive more charge and, therefore, more current from a power supply. Thus, when using high capacitance capacitors, for example, double-layer capacitors, high current draw needs to be considered during the design-in phase even more than before. It would also be preferable to avoid extensive start-up delays that use of high capacitance capacitors may cause.
Second, in some applications excessive current draw may disable the power supply. For example, large current drawn from a power supply can blow a fuse, trip an overload protection circuit, or cause permanent damage to internal components of the power supply. Excessive current draw may also damage the capacitor, causing it to leak, catch fire, or even explode, presenting a safety hazard. Therefore, it would be desirable to prevent excessive current draw and avoid such possibilities.
Third, a typical capacitor failure mode is a short circuit between capacitor terminals. With the capacitor installed across power supply output terminals, the failure would not only affect the AC noise suppression and ride-through capability of the power supply, but would also cause a catastrophic failure because the voltage level output by the power supply would likely fall precipitously, leaving the equipment powered by the supply without adequate power. It would be beneficial to prevent such catastrophic failures due to capacitor failures.
A need thus exists for methods and apparatus to prevent excessive start-up delays caused by charging output capacitors of power supplies. Another need exists to prevent excessive current draw that can disable power supplies during equipment start-up. Yet another need exists to prevent capacitor failures from causing catastrophic equipment failures. A further need exists to implement such solutions with high capacitance capacitors such as double-layer capacitors.